1. Field of the Invention
The present invention relates to a light-signal delaying device in which a required amount of delay is given to each channel to correct the amount of delay of a light signal transmitted in parallel in each channel in an optical multiplex transmission system, particularly, in a multiple-wavelength multiplex high-speed transmission system. More specifically, the present invention concerns a light-signal delaying device in which the delay can be finely adjusted over a wide range and effects on the optical transmission system can be minimized.
2. Description of the Related Art
Several systems have been proposed as conventional light-signal delaying devices for use in optical ATM exchanges or the like. However, in most cases, the delay time is adjusted over a small range. For adjusting the delay time over a relatively wide range, an optical fiber collimator is generally used. Referring to FIG. 5, a conventional light-signal delaying device (disclosed in Japanese Unexamined Patent Application Publication No. 11-295529) will be described. In FIG. 5, reference numerals 501 and 502 denote first and second collimating lenses, respectively; reference numeral 503 denotes an input-side optical fiber; reference numeral 504 denotes an output-side optical fiber; and reference numeral 505 denotes a support block. A moving shaft 508 is rotated by a moving mechanism 506. A base 507 supports the moving shaft 508 and the first collimating lens 501. The symbol xcex94L indicates the amount of change in the distance between the first collimating lens 501 and the second collimating lens 502.
In the light-signal delaying device shown in FIG. 5, collimated light is directly transmitted between a pair of optical collimators formed of the pair of first and second optical fibers 503 and 504 and the respective lenses 501 and 502. When the distance xcex94L between the collimating lenses 501 and 502 is adjusted, the transmission time of an incoming and outgoing light signal can be delayed by moving the second collimating lens 502 along the optical axis by the moving mechanism 506, thus changing the distance between the collimating lens 501 and the collimating lens 502. In such a system, since the moving optical fiber 504 and the second collimating lens 502 move as the delay time is adjusted, a stress is exerted on the connection of the parts and the moving fiber 504. As a result, problems such as undesired positional deviation and damage to the moving fiber 504 may occur. Also, since it is not possible to avoid meandering during movement or positional deviation in the lateral direction in a driving mechanism having only the moving shaft 508, it is difficult to sufficiently reduce the misalignment between the optical axes of the opposed optical fiber collimators. Also, since an increase in insertion loss arising from insufficient control of the misalignment between the optical axes is inevitable, adjustment of the delay over a wide range is next to impossible.
FIG. 6 shows an example of the optical path in a conventional light-signal delaying device. In FIG. 6, reference numerals 601 and 602 denote a first optical fiber and a second optical fiber, respectively. First and second optical collimators 603 and 604 are arranged so as to correspond to the first and second optical fibers 601 and 602, respectively. A rectangular prism 605 can be moved in the direction shown by the arrow in FIG. 6. The symbol xcex94L indicates a geometric variable distance between the incidence/exit end face of the rectangular prism 605 and the optical collimators 603 and 604.
In the light-signal delaying device shown in FIG. 6, since the light beam is transmitted through the rectangular prism 605 between the optical collimators 603 and 604, the light beam is reflected by an incidence boundary surface 605a and an exit boundary surface 605b of the rectangular prism 605. Therefore, losses due to Fresnel reflection at the incidence/exit boundary surfaces 605a and 605b are inevitable. The Fresnel loss is approximately 0.6 dB (=0.3 dBxc3x972). Furthermore, when the reflected light returns to the light-signal transmission system via the first optical collimator 603, the luminous output becomes unstable, thereby causing adverse effects such as an increase in distortion or an error rate in the light-signal waveform. Also, in the light-signal delaying device shown in FIG. 6, while the rectangular prism 605 needs to be moved along the optical axis, a movement guide mechanism is not specified and an insertion loss is 2.5 dB according to reference literature. Accordingly, the configuration of the movement guide mechanism is unsatisfactory.
As described above, according to the conventional light-signal delaying device, it is extremely difficult to achieve adjustment of the delay over a wide range, a compact configuration, and low insertion-loss characteristics.
Accordingly, it is an object of the present invention to provide a compact light-signal delaying device in which a transmitted light signal can be given a desired amount of delay over a wide range and the insertion loss can be reduced, thereby producing no adverse effects on the optical transmission system.
In order to achieve the above objects, a light-signal delaying device according to the present invention includes a linear guide rail, a pair of stages engaged with the linear guide rail, a V-groove holder mounted on one of the stages, a pair of optical fiber collimators secured to the V-groove holder, a pair of reflecting mirrors mounted on the other stage in such a manner that reflecting surfaces thereof intersect at 90xc2x0, and an actuator for moving the stage on which the reflecting mirrors are mounted along the linear guide rail.
In the light-signal delaying device according to the present invention, one of the pair of stages to which the pair of optical fiber collimators is fixed is a fixed stage that is fixed at a predetermined position on the linear guide rail, the stage to which the pair of reflecting mirrors is fixed is a movable stage capable of moving along the linear guide rail, and each stage has a pressing mechanism for applying pressure in a direction perpendicular to the direction of movement of the linear guide rail to prevent a gap between the stages and the linear guide rail.
In the light-signal delaying device according to the present invention, each of the pair of optical fiber collimators is a GRIN lens having a pitch of 0.25, which is connected to an optical fiber, connecting end faces of the GRIN lens and the optical fiber are obliquely polished at 6xc2x0 or more and connected to each other, and an anti-reflection film is formed on each light-beam incidence/exit end face.
In the light-signal delaying device according to the present invention, the pair of reflecting mirrors is a first reflecting mirror and a second reflecting mirror, which are arranged in such a manner that the reflecting surfaces meet each other at 90xc2x0 and have an incidence angle and an exit angle of 45xc2x0 with respect to incident light and exit light, respectively, the first reflecting mirror deflects the light beam incident from the first optical fiber collimator by 90xc2x0 with respect to the optical axis, and reflects it toward the second reflecting mirror, and the second reflecting mirror deflects the incident light beam by 90xc2x0 with respect to the optical axis, and reflects it along the optical axis of the second optical fiber collimator.
In the light-signal delaying device according to the present invention, the V-groove holder has two V-grooves formed in parallel, the pair of optical fiber collimators is arranged in the V-groove holder such that the optical axes are in parallel with each other, the pair of reflecting mirrors is secured to the movable stage such that each of them faces the corresponding one of the pair of optical fiber collimators, and the light beam propagating between the optical fiber collimators can be sent to and received from the reflecting mirrors with a low transmission loss.
In the light-signal delaying device according to the present invention, the actuator for moving the movable stage is based on any one of a motor-driven ball screw mechanism, an air-driven cylinder mechanism, and a manual ball screw mechanism.
In the light-signal delaying device according to the present invention, each of the pair of reflecting mirrors has a metal film or a multilayer dielectric film formed on the reflecting surface thereof.
In the light-signal delaying device according to the present invention, each optical fiber used in the optical fiber collimators is any of a single mode fiber, a mode-dispersion shift fiber, and a polarization preserving fiber.